Enable -Znext-solver on nightly by default

[Kivooeo]

View all comments

Implementation of rust-lang/compiler-team#1014 cc rust-lang/blog.rust-lang.org#1896 #160895

This enables -Znext-solver=globally by default in nightly, but keeps -Znext-solver=coherence for tests

[Kobzol]

Note that #161273 is also approved, and it also enables a new unstable feature on nightly by default. While it should be much less invasive as the next trait solver, it can in theory also cause some breakages. I think that we shouldn't land both in the same nightly.

I'm fine with giving this PR more priority if you want, though we already put out a blog post about the Cargo change (https://blog.rust-lang.org/inside-rust/2026/08/18/reducing-target-dir-size-on-nightly/).

[Kivooeo]

ehh

[Kobzol]

@bors r-

[rust-bors]

This pull request was unapproved.

View changes since this unapproval

[lcnr]

Note that #161273 is also approved, and it also enables a new unstable feature on nightly by default. While it should be much less invasive as the next trait solver, it can in theory also cause some breakages. I think that we shouldn't land both in the same nightly.

as in, merge next-solver tomorrow instead? Seems fine to me. One day more or less doesn't matter much and means we don't have to rush as much here

[rust-bors]

☀️ Try build successful (CI)
Build commit: f8af1d1 (f8af1d1bd615f5828f73361441d316a4b4b63c40)
Base parent: 444dcce (444dcce484c66c8d626806a8556be3b4d3ef381a)

[Kobzol]

Note that #161273 is also approved, and it also enables a new unstable feature on nightly by default. While it should be much less invasive as the next trait solver, it can in theory also cause some breakages. I think that we shouldn't land both in the same nightly.

as in, merge next-solver tomorrow instead? Seems fine to me. One day more or less doesn't matter much and means we don't have to rush as much here

Yup. Btw the PR CI failure looks real.

[rust-timer]

Finished benchmarking commit (f8af1d1): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	3.8%	[0.2%, 14.5%]	214
Regressions ❌ (secondary)	3.1%	[0.1%, 14.9%]	255
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-7.8%, -0.6%]	26
All ❌✅ (primary)	3.8%	[0.2%, 14.5%]	214

Max RSS (memory usage)

Results (primary 3.4%, secondary 4.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.4%	[0.5%, 7.9%]	190
Regressions ❌ (secondary)	4.6%	[0.8%, 10.1%]	225
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-2.6%	[-4.3%, -0.9%]	21
All ❌✅ (primary)	3.4%	[-1.0%, 7.9%]	191

Cycles

Results (primary 6.3%, secondary 5.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	6.3%	[2.2%, 12.7%]	134
Regressions ❌ (secondary)	5.9%	[1.6%, 13.7%]	129
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-4.3%, -2.4%]	3
All ❌✅ (primary)	6.3%	[2.2%, 12.7%]	134

Binary size

Results (secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 455.347s -> 467.92s (2.76%)
Artifact size: 398.98 MiB -> 396.32 MiB (-0.67%)

[Kobzol]

The perf. result should be neutral, so something is wrong somewhere.

[Kobzol]

Note that rustc-perf only passes cargo_rustc_opts to the final leaf crate being compiled. Is it OK if all dependencies are compiled using the old trait solver, but then the leaf crate is compiled using the next trait solver?

This is a Cachegrind diff for the diesel/check benchmark:

< 15,313,179,386  ???:
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     -16,191,251    thin_vec::drop_non_singleton::<rustc_infer::traits::Obligation<rustc_middle::ty::predicate::Predicate>>

[lcnr]

blocked on #161344